Oversampling analog-to-digital converters generally consist of a modulator section which receives an analog signal and provides a serial data stream having a bit rate which is much greater than the Nyquist sampling frequency, followed by a digital filtering and decimation section which provides at its output a digitized representation of the analog input signal. Inside the modulator is an analog loop filter which is coupled to a summing node at its input and which provides an output that is digitized by a one bit analog-to-digital converter. The output of the one bit analog-to-digital converter forms the output of the modulator. The summing node sums the analog input signal with the output of the one bit analog-to-digital converter to provide an error signal which is input to the analog loop filter.
In the past, the analog loop filter has usually been either a first order, a second order, a third order, or a fourth order filter which uses either all fully continuous time integrators or all discrete time integrators. For example, U.S. Pat. No. 4,509,037 to R. W. Harris describes a Delta modulation encoder which uses a third order analog loop filter comprised of continuous time integrators, and U.S. Pat. No. 4,746,899 to E. J. Swanson et al., describes a second order analog loop filter which uses discrete time integrators.
Both the continuous time integrator and the discrete time integrator have their own advantages and disadvantages. For example, the continuous time integrator, while generally requiring external components, can provide low thermal noise characteristics. Moreover, since the continuous time integrator is continuous, there is no introduction of the sampling noise into the loop. While the prior art discrete time integrators introduce the sampling frequency noise into the loop and require anti-aliasing filters, and while they cannot easily match the thermal noise characteristics of the continuous time integrator in a practical circuit, the discrete time integrator is able to provide much more tightly controlled loop characteristics to thereby enable stability in higher order analog loop filters such as the fourth order filter.
Therefore it can be appreciated that an analog loop filter which is able to combine the favorable characteristics of a continuous time integrator and a discrete time integrator while avoiding the major disadvantages of each type of integrator is highly desirable.